1. Field of the Invention
This invention relates to a variable impedance circuit of a type suitable for use for a wide variety of circuits such as in analog computers, AGC circuits, four channel stereophonic equipment, etc. For this purpose, a conventional FET, CdS photo-conductive impedance element, etc. are known but to maintain relatively low distortion, the amplitude of input signal must be small or less than some relative low predetermined level. This means S/N ratio has a predetermined ceiling value which will be required to be increased more and more in view of Hi-Fi equipment, for example, which requires low noise level.
2. Description of the Prior Art
Semiconductor devices having a resistive layer overlying an insulating layer on a semiconductor substrate are known in the prior art. One disclosure of such a device is a field effect transistor described in U.S. Pat. No. 3,714,522. A device of this type is now referred to in the industry by the term "RIS", which stands for resistive insulating semiconductor device. In using an RIS field effect transistor in the past, there has been trouble in maintaining a relatively low distortion over a wide range of frequencies. The present invention provides a circuit employing an RIS field effect transistor which for the circuit, gives extremely good linearities and low distortion over a wide range of frequencies.